1. Field of the Invention
This invention relates to an improved process for selective removal of H.sub.2 S from gaseous mixtures containing H.sub.2 S and CO.sub.2 wherein solutions of a class of heterocyclic tertiary aminoalkanols are employed.
2. Description of Related Patents
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The treatment of acid gas mixtures containing, inter alia, CO.sub.2 and H.sub.2 S with amine solutions typically results in the simultaneous removal of substantial amounts of both the CO.sub.2 and H.sub.2 S. For example, in one such process generally referred to as the "aqueous amine process," relatively concentrated amine solutions are employed. A recent improvement on this process involves the use of sterically hindered amines as described in U.S. Pat. No. 4,112,052, to obtain nearly complete removal of acid gases such as CO.sub.2 and H.sub.2 S. This type of process may be used where the partial pressures of the CO.sub.2 and related gases are low. Another process often used for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., H.sub.2 S, COS, CH.sub.3 SH and CS.sub.2 involves the use of an amine in combination with a physical absorbent, generally referred to as the "nonaqueous solvent process." An improvement on this process involves the use of sterically hindered amines and organic solvents as the physical absorbent such as described in U.S. Pat. No. 4,112,051.
It is often desirable, however, to treat acid gas mixtures containing both CO.sub.2 and H.sub.2 S so as to remove the H.sub.2 S selectively from the mixture, thereby minimizing removal of the CO.sub.2. Selective removal of H.sub.2 S results in a relatively high H.sub.2 S/CO.sub.2 ratio in the separated acid gas which simplifies the conversion of H.sub.2 S to elemental sulfur using the Claus process.
The typical reactions of aqueous secondary and tertiary amines with CO.sub.2 and H.sub.2 S can be represented as follows: EQU H.sub.2 S+R.sub.3 N.revreaction.R.sub.3 NH.sup.+ +SH.sup.- ( 1) EQU H.sub.2 S+R.sub.2 NH.revreaction.R.sub.2 NH.sub.2.sup.30 +SH.sup.31 ( 2) EQU CO.sub.2 +R.sub.3 N+H.sub.2 O.revreaction.R.sub.3 NH.sup.30 +HCO.sub.3.sup.31 ( 3) EQU CO.sub.2 +2R.sub.2 NH.revreaction.R.sub.2 NH.sub.2.sup.+ +R.sub.2 NCOO.sup.31 ( 4)
wherein R is an organic radical which may be the same or different and may be substituted with a hydroxyl group. The above reactions are reversible, and the partial pressures of both CO.sub.2 and H.sub.2 S are thus important in determining the degree to which the above reactions occur.
While selective H.sub.2 S removal is applicable to a number of gas treating operations including treatment of hydrocarbon gases from shale pyrolysis, refinery gas and natural gas having a low H.sub.2 S/CO.sub.2 ratio, it is particularly desirable in the treatment of gases wherein the partial pressure of H.sub.2 S is relatively low compared to that of CO.sub.2 because the capacity of an amine to absorb H.sub.2 S from the latter type gases is very low. Examples of gases with relatively low partial pressures of H.sub.2 S include synthetic gases made by coal gasification, sulfur plant tail gas and low-Joule fuel gases encountered in refineries where heavy residual oil is being thermally converted to lower molecular weight liquids and gases.
Although it is known that solutions of primary and secondary amines such as monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA), and hydroxyethoxyethylamine (DGA) absorb both H.sub.2 S and CO.sub.2 gas, they have not proven especially satisfactory for preferential absorption of H.sub.2 S to the exclusion of CO.sub.2 because the amines undergo a facile reaction with CO.sub.2 to form carbamates.
Diisopropanolamine (DIPA) is relatively unique among secondary aminoalcohols in that it has been used industrially, alone or with a physical solvent such as sulfolane, for selective removal of H.sub.2 S from gases containing H.sub.2 S and CO.sub.2, but contact times must be kept relatively short to take advantage of the faster reaction of H.sub.2 S with the amine compared to the rate of CO.sub.2 reaction as shown in Equations 2 and 4 hereinabove.
In 1950, Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950) showed that the tertiary amine, methyldiethanolamine (MDEA), has a high degree of selectivity toward H.sub.2 S absorption over CO.sub.2. This greater selectivity was attributed to the relatively slow chemical reaction of CO.sub.2 with tertiary amines as compared to the rapid chemical reaction of H.sub.2 S. The commercial usefulness of MDEA, however, is limited because of its restricted capacity for H.sub.2 S loading and its limited ability to reduce the H.sub.2 S content to the level at low pressures which is necessary for treating, for example, synthetic gases made by coal gasification.
Recently, U.K. Patent Publication No. 2,017,524A to Shell disclosed that aqueous solutions of dialkylmonoalkanolamines, and particularly diethylmonoethanolamine (DEAE), have higher selectivity and capacity for H.sub.2 S removal at higher loading levels than MDEA solutions. Nevertheless, even DEAE is not very effective for the low H.sub.2 S loading frequency encountered in the industry. Also, DEAE has a boiling point of only 161.degree. C.; therefore it is characterized as being a low-boiling, relatively highly volatile amino alcohol. Such high volatilities under most gas scrubbing conditions result in large material losses with consequent losses in economic advantages.
Other absorbent solutions which are selective to H.sub.2 S removal but have selectivity and/or capacity limitations similar to those of MDEA include solutions of 1-formylpiperidine as described in U.S. Pat. No. 4,107,270, piperazine as described in U.S. Pat. No. 4,112,049, and solutions, in N-methyl-2-pyrrolidone, of cyanopyridine, an aromatic nitrile, an anhydride, or a piperazine, as described in U.S. Pat. Nos. 3,656,887, 3,767,766, 3,642,431, and G. B. Pat. No. 1,560,905, respectively.